With the arrival and expansion of the Information Highway the quantity and speed of data transmission continues to grow. Telephone networks have being slowly converted from an all analog environment to a virtually all-digital network. Within these networks, the trunks and switches have been virtually 100 percent converted, while the local loops leading to the customer remain largely analog, other than in those environments where Integrated Services Digital Network (ISDN) technology has managed to gain a foothold.
This evolution towards a completely digital network brings many advantages, including the ability to provide a broader and more diverse range of services, a less error-prone network, the ability to achieve both physical and logical integration of applications, and an enhanced ability to deliver the higher bandwidth services requested by many customers.
Unfortunately, this deployment of higher bandwidth services must overcome a large issue, notably the fact that the traditional telephone network is designed to transport voice and relatively low-speed data, as opposed to high-speed digital data. It has also been designed around the known behavior patterns of typical telephony users, with well-understood calling patterns and hold times. The growth in Internet access has had an important, even negative effect on the ability of the local switching infrastructure to handle the requested load, since most users log on and stay connected much longer that the typical telephone call, which is what the switches were designed to handle.
Solutions to the bandwidth bottleneck problem faced by the telecommunications networks include massive switch capacity retrofits, construction of overlay networks for Internet traffic, as well as schemes designed to limit customers' connect time. A particularly promising solution is of a technological nature, specifically the use of Asymmetric Digital Subscriber Line (ADSL) in the networks' analog local loops. ADSL is a technology that offers the subscribers enormous bandwidth, engineered to overlay the existing analog Plain Old Telephone Service (POTS) and basic rate ISDN service. The term"asymmetric" arises from the fact that the system offers as much as 6 Mbps toward the customer, as compared to 384 Kbps from the customer.
Within traditional telephone networks, a copper loop (or two-conductor cable) connecting the central office and the CPE is used to provide the POTS, whose signals are Voice Frequency (VF) signals in the frequency range of 0-4 KHz. These cables are capable of carrying signals up to several MHz, depending on their length and type. ADSL takes advantage of the unused frequency band above 4 KHz and uses the copper loop as its transmission medium, transporting voice in the traditional 4 KHz channel bandwidth where it has always been, while higher bandwidth digital services are relegated to higher frequency domains.
In order to overlay the existing analog POTS, the ADSL system includes, among others, two circuits: a POTS Splitter circuit and a 2-4 W (2 wire to 4 wire) Hybrid circuit. These two circuits are used to combine and separate the POTS and ADSL signals, and are needed at both the CPE and central office ends of the copper analog loop. Unfortunately, the typical ADSL POTS Splitter and 2-4 W Hybrid circuitry is not only expensive, large and very power inefficient, but also produces important distortion which limits the cable range between CPE and central office in certain cases.